Magnetic elements such as inductors or transformers are widely used in switch-mode power converters. The magnetic element is a key component influencing power density, efficiency and reliability of the power converter. Conventionally, the magnetic element (e.g. an inductor) used in the switch-mode power converter is made of ferrite, ring-shaped powder core, or the like. Since different magnetic core materials have different hysteresis properties, the losses of different magnetic cores are distinguished. Generally, the magnetic core loss is resulted from alternate magnetic fields within the magnetic core. The magnetic core loss is a function of the operating frequency and the total magnetic flux swing (ΔB). The magnetic core loss usually includes hysteresis loss, eddy-current loss and residual loss. As the permeability is increased, the hysteresis curve becomes narrower and the power consumption of the magnetic core is reduced. The magnetic core made of ferrite is cost-effective and has low power consumption of the magnetic core. Since the saturation flux density of ferrite is low, an air gap and a Litz wire are necessary. In such situation, the overall volume is relatively huge. On the other hand, the magnetic core made of ring-shaped powder core has higher saturation flux density and may store larger amount of energy. The process of fabricating an inductor by using the ring-shaped powder core needs a manual winding step, and thus the fabricating process is time-consuming. For simplifying the winding step of the fabricating process, the advantages of the ferrite and ring-shaped powder core combined together in the practical applications.
According to the magnetic path designs, the above two materials are combined together by either connected in parallel or in series. In a case that the two materials are combined together in parallel, the functions of these two materials are added but the overall volume is increased. In a case that the two materials are combined together in series, the functions of these two materials are moderate but the overall volume is reduced.
For preventing core saturation and minimizing eddy-current loss, U.S. Pat. No. 6,980,077 disclosed a method of filling the air gap of the magnetic path by using a magnetic powder core. This method is applied to ferrite EI or EE magnetic core assembly. The magnetic path is increased by filling the air gap with the magnetic powder core. In practice, for maintaining the original anti-saturation property, the length of the magnetic powder core should be extended. By the calculating method disclosed in this patent, the magnetic powder core (having the same permeability as the current standard magnetic powder core) is usually longer than the center legs of the EI and EE magnetic core assemblies. In other words, the application thereof is largely restricted. In a case that the permeability of the magnetic powder core is further reduced, the fringing flux is increased and a near-field radiation problem occurs.
U.S. Pat. No. 7,265,648 disclosed a method for achieving nonlinear inductance by using a high permeability material. In this method, two magnetic core members (one of them has an air gap) are connected in parallel. In practice, the magnetic core member with the air gap may incur near-field radiation, electromagnetic interference and high eddy-current loss. In a case that a ferrite magnetic core and an alloy magnetic powder core are parallel, better performance is achieved. This patent, however, fails to obviate the above drawbacks encountered from the prior art.
U.S. Pat. No. 5,062,197 disclosed a method of providing a high-frequency inductor or transformer by using two magnetic materials. This method uses too many components and is very complicated. Since the center leg is made of a high saturation flux density and low permeability material (e.g. ferrite), the cross-section area and the mean turn length are increased. In this situation, the resistance is increased. In addition, the two low-permeability layers within the magnetic core incur a large magnetic pressure distribution. As such, the near-field radiation and electromagnetic interference problems are incurred.
From the above discussions, it is found that the conventional magnetic elements fail to effectively increase the operating efficiency, shortening the fabricating time or reducing the cost and overall volume of the magnetic core assembly. For obviating the drawbacks encountered from the prior art, there is a need of combining two magnetic core materials and quickly assembling a magnetic element in a simplified manner.